1. Field of the Invention
The invention relates to the production of synthetic gas, and more particularly to a method and a system for producing synthetic gas from biomass by pyrolysis. The method belongs to the technical field of producing synthetic gas or combustible gas by using biomass. The synthetic gas is a mixture gas which contains CO, H2, and a variety of carbohydrates that contain carbon, hydrogen, and oxygen. The synthetic gas produced by the method according to the invention can be used for gas turbine power generation systems, fuel cells, synthetic oil, metallurgical and other systems.
2. Description of the Related Art
As dwindling of traditional fossil fuels (coal, oil, and natural gas) reserves and the environmental pollution problems caused by the use of fossil fuels directly threaten human survival and development, attaching importance to development of renewable and environmental friendly energy has become a consensus of governments of all countries. Biomass, an organic matter generated by plants through photosynthesis, has wide sources and large available quantity. It can be transformed into clean gas or liquid fuel for power generation and producing industrial raw materials and chemical products. As energy it is clean and renewable with zero emission of carbon dioxide and with the potential to fully replace fossil fuels as a new energy resource which has become a priority for all countries.
There are many methods for transforming biomass into clean gas or liquid fuel, among which biomass gasification technology can adapt to a variety of species and has good expansibility. The gasification of biomass is a thermochemical process, i.e., biomass reacts with a gasification agent (such as air, oxygen, vapor, carbon dioxide, etc.) under high temperature to produce a mixed gas consisting of carbohydrate containing carbon, hydrogen, and oxygen. The mixed gas is named synthetic gas. The components of the synthetic gas are decided by the species of used biomass, the type of the gasification agent, the reaction conditions, and the structure of a gasifier used therein. The objectives of gasification is, on the one hand, to minimize the consumption of materials and the gasification agent, as well as the tar content in the synthesis gas, and on the other hand, to maximize the gasification efficiency and the efficiency of carbon conversion, as well as the active ingredient (CO and H2) content in the synthesis gas. The objectives are decided by the type of the used gasifier, the type of the gasification agent, the particle size of the biomass, the gasification pressure and temperature, and moisture and ash of the biomass, etc.
The gasification furnace used in the gasification process can be divided into three classes: fixed bed, fluidized bed, and entrained flow bed. The fixed bed has a simple gasification structure, convenience operation, flexible operation mode, a higher rate of carbon conversion, a wide range of running load which can be between 20% and 110%, and the solid fuel stays in the bed for a long period of time. However, the temperature is nonuniform and it has less efficiency of heat exchange, low heating value of the synthesis gas at the outlet, and the synthesis gas contains a large amount of tar. The fluidized bed is convenient for material addition and ash release, and the temperature is uniform and easy for adjustment. However, it is sensitive to the characteristics of raw materials. If the adhesion, thermal stability, moisture content, or ash melting point of raw materials changes, the operation will become abnormal. Furthermore, in order to ensure normal fluidization of the gasification furnace, it needs to keep lower temperature, and the synthetic gas has a large amount of tar. Since a large amount of tar is produced in the fixed bed and the fluidized bed, a tar cracking unit and purification equipment must be installed, which results in a complicated process. The entrained flow bed has a high and uniform operating temperature, good amplification characteristics, and particularly suitable for large-scale industrialization. Tar is cracked completely. However, the entrained flow bed has a strict requirement on particle size of raw materials. Based on current grinding technology, there is no way to grind biomass having much cellulose to a size suitable for the entrained flow bed. So the entrained flow bed cannot be used for gasification of biomass. Nowadays, tar cracking and pretreatment of biomass prior to gasification are tough problems for the development of biomass gasification.
A typical method for gasifying low tar biomass is summarized below. The method includes pyrolysis and gasification independently, and biomass is transformed into synthetic gas containing low content of tar. In the method, pyrolysis gas and charcoal experience incomplete combustion in the gasifier at around 1000° C., and tar is cracked under high temperature. Although the tar content is decreased greatly, a lot of charcoal is consumed, resulting in a low content of CO produced in the subsequent reduction reaction and a high content of CO2 in the synthetic gas. Secondly, due to a low temperature in the combustion reaction, the temperature at the subsequent reduction becomes lower, and the average temperature in the reduction zone is less than 700° C., and thereby the yield of effective synthetic gas (CO and H2) is decreased significantly (about 30%). Thirdly, the ash and unreacted carbon residue from the reduction reaction is directly discharged, resulting in a low carbon conversion rate. Finally, the gasifier used in the method is in the form of a fixed bed, since the reduction reaction absorbs heat, the temperature difference between the top and the bottom (the top is about 1000° C. and the bottom is about 500° C.) of the bed is huge, which is an inherent disadvantage of fixed bed.
A typical method for producing synthetic gas with carbon-containing materials is described below. The method includes carbonization (or pyrolysis) and gasification independently. In the method, the carbonization temperature is controlled less than 450° F. so as to reduce the tar content resulted from pyrolysis. However, during carbonization stage, solid products are not ground prior to transporting to the reaction coils of the gasifier, which will affect the speed and degree of gasification reaction. Secondly, since the gasification reaction happens in the reaction coil, a large amount of transport gas is needed, but the transport gas will take away a lot of heat during transporting, and thereby the gasification efficiency is low, the temperature is nonuniform, and the subsequent waste heat recovery system is massive. Thirdly, it is not economic that newly-produced synthetic gas is used to provide heat for gasification and carbonization. Fourthly, combustion products (mainly CO2 and H2O) are directly discharged and not fully utilized, resulting in low gasification efficiency. Finally, the ash and unreacted carbon residue in the synthetic gas are also discharged directly, resulting in low carbon conversion rate.
Another typical method for producing synthetic gas from biomass by high temperature gasification is as follows. The method also adopts combination of carbonization and high temperature gasification. However, the method has following problems: firstly, heat of the carbonization furnace is supplied by direct combustion of external combustible gas and oxygen; the introduced high-quality external fuel gas greatly increases the energy consumption of the system; secondly, the adopted pyrolysis gas powder feeding system is complicated; when the high temperature pyrolysis gas is mixed with the low temperature carbon powder and fed into the gasification furnace, the mixture can easily be condensed to form tar, causing blockage and influencing the normal operation; finally, the high pressure charcoal produced in the carbonization furnace is fed into the normal pressure milling machine after being decompressed and cooled, so as to be made into powder, and then the carbon powder is pressurized and fed into the gasification furnace by the pyrolysis gas. The whole process is complicated and high in energy consumption so that the feasibility of the project is bad.
From the above mentioned methods, conventional gasification, whether from biomass or from solid carbon-containing materials, cannot produce synthetic gas with high efficiency and low cost. Although the technology of independent pyrolysis and gasification can adapt to a variety of biomass and reduce the content of tar in synthetic gas, shortcomings such as nonuniform temperature, large investment in equipment for waste heat recovery, high material consumption, low gasification efficiency, and low carbon conversion rate limit the application of biomass gasification in industry. Particularly, there is no effective method for gasifying biomass applied to an entrained flow bed.